Acidity measures how much acid something contains. Scientists describe acidity using a scale called pH, where lower numbers mean more acidic. Lemon juice, vinegar, and orange juice all have high acidity, which gives them that sharp, sour, tangy taste that makes your mouth pucker.

In chemistry class, you might test a liquid's acidity using special paper that changes color. Pure water sits right in the middle of the pH scale, neither acidic nor basic. Acids can be powerful: the acid in your stomach breaks down food during digestion, while acid rain (caused by pollution) can damage buildings and harm forests.

When cooking, understanding acidity matters because acidic ingredients like tomatoes or citrus juice can brighten flavors and balance richness. A squeeze of lemon juice adds acidity to cut through the heaviness of fried fish.

Scientists and doctors also talk about acidity in soil (which affects what plants can grow), in oceans (where increasing acidity threatens coral reefs), and in the human body (where the right acid balance keeps you healthy). When something has a lot of acid, we say it has high acidity.